The present invention relates to a process for the manufacture of double-sided or multilayer printed circuit boards with printed plated resistors. The method proposed produces printed circuits with integral resistors, which are printed and plated in place on the surfaces of the printed circuit board, or on the inner cores of multilayer printed circuit boards thereby opening the area on the surface of the board for placement of active devices. The process produces printed circuit boards with resistors in a manner that is more efficient and economical than previously possible.
In the manufacture of printed circuits, it is now commonplace to provide planar boards having circuitry on each side thereof (e.g. double-sided circuit boards). It is also commonplace to produce boards comprised of integral planar laminates of insulating substrate and conductive metal, wherein one or more parallel innerlayers or planes of the conductive metal, separated by insulating substrate, are present within the structure, with the exposed outer surfaces, along with the inner planes, of the laminate containing printed circuit patterns (e.g. multilayer circuit boards).
In double sided and multilayer circuit boards, it is necessary to provide interconnection between or among the various layers and/or sides of the board containing the conductive circuitry. This is achieved by providing metalized, conductive thru-holes in the board communicating with the sides and layers requiring electrical interconnection. The predominantly employed method for providing conductive thru-holes is by electroless deposition of metal on the non-conductive surfaces of the thru-holes, which have been drilled or punched through the board. Typically the electroless deposition is followed by electrolytic deposition of metal in the holes to build conductive metal to the required thickness. Recently some processes have allowed for direct electroplating in the thru-holes without need for prior electroless deposition.
The typical manufacturing sequence for producing printed circuit boards begins with a copper-clad laminate. The copper clad laminate comprises a glass reinforced epoxy insulating substrate with copper foil adhered to both planar surfaces of said substrate, although other types of insulating substrates such as paper phenolic and polyimide, have been used. First the thru-holes are drilled or punched in the copper clad laminate thereby exposing the hole surfaces of insulating substrate material. The holes are then subjected to a chemical plating process which deposits conductive metal in the holes as well as on the copper surfaces. A plating mask is provided on the outer surfaces in the negative image of the circuitry desired. Subsequently copper is electroplated on all surfaces, not covered by the plating mask, to a predetermined thickness, followed by a thin deposition of tin to act as an etch resist. The plating resist is then stripped and the exposed copper surfaces (i.e., those not plated with the etch resist) are etched away. Finally the etch resist is removed and printed circuit board is finished with one of a number of known finishing methods such as solder mask, followed by hot air solder leveling. The foregoing process is typically called the pattern plate process and is suitable for producing double-sided printed circuit boards or multilayer boards. However, in the case of multilayer boards the starting material is a copper clad laminate which comprises inner planes of circuitry called innerlayers.
Simple printed circuit boards and the innerlayers of a multilayer circuit board are produced through a technique called print and etch. In this manner a photopolymer is laminated or dried on the copper surfaces of a copper clad laminate. The photopolymer is then selectively imaged using a negative and developed to produce a positive image of the desired circuit pattern on the surfaces of the copper clad laminate. The exposed copper is then etched away and the photopolymer stripped, revealing the desired circuit pattern.
The semi-additive process may be used in conjunction with the print and etch process to produce double sided or multilayer print and etch boards with plated thru-holes. In this process a copper clad laminate or a multilayer package with copper foil on the exterior surfaces is processed through the print and etch process as given above. Holes are then drilled in the board in a desired array. A plating resist applied to cover substantially the entire outer surfaces of the board except for the holes and the circuits. Typically, a separate desensitizing mask is applied, the holes are activated and the desensitizing mask is then stripped away without disturbing the activation. The exposed areas are then plated electrolessly.
In addition to the foregoing, many other processes have been utilized to produce printed circuit boards. Some of these processes are detailed in U.S. Pat. Nos. 3,982,045, 4,847,114 and 5,246,817, the teachings each of which are incorporated herein by reference in their entirety. However, in the prior art processes, the circuits are made such that resistors, if required, need to be provided externally from the circuit board itself. (e.g. mounted on the surface of the circuit board as an appendage).
A process whereby reliable resistors can be printed and plated as an integral part of the circuitry of the printed circuit board is disclosed herein. This provides for an efficient and economical way of providing the necessary resistors. In addition the process provides for further miniaturization of the printed circuit boards produced in comparison to those produced by prior art methods. Typical prior art in this regard are U.S. Pat. Nos. 3,808,576 and 2,662,957, the teachings both of which are incorporated by reference herein in their entirety. This invention produces printed circuits with integral resistors, which resistors have a particularly constant resistance as is required by the most demanding applications.